Pena has been absolutely abominable as a pinch hitter over the course of his career. It isn’t a particularly large sample size, but striking out in nearly half of his plate appearances is pretty remarkable … it’s like he’s facing the 1964 version of Sandy Koufax every time he comes to the plate. Most successful pinch hitters are punch-and-judy types like Lenny Harris who have good bat control and put the ball into play. Pena is exactly the opposite of that profile.
Now, pinch hitting doesn’t really excuse Pena’s performance this year, since the Red Sox don’t pinch hit much, and he’s only had a handful of at-bats in that role. By extension, however, I would guess that Pena is the sort of hitter who needs regular work to keep his hitting mechanics in order. If he comes into the game cold after a couple of days on the bench, and feels like he has to fight for every at-bat, he’s going to press, and that’s especially problematic for a guy with some holes in his swing.
Frankly, Washington is probably a better fit for Wily Mo. Players from this template (see also Hee Seop Choi) may not work particuarly well off the bench, but they might also not provide quite enough value to be regulars for a first-division club. Pena is the baseball equivalent of someone like Shareef Abdur-Rahim — he needs a lot of shots to be effective, but if he’s your best scorer, you probably aren’t going to the playoffs. Thus, he’s pretty much always going to wind up on a lottery club. Is it any coincidence that Dave Kingman appeared in just one post-season series in a 17-year career?